The present invention relates to a voltage conversion circuit and a differential difference amplifier (DDA) containing a voltage conversion circuit and, more particularly, to a voltage conversion circuit and differential difference amplifier suitable for a CMOS integrated circuit using a CMOS circuit.
A DDA is an amplifier for outputting a voltage proportional to the difference between two differential input voltages. A conventional instrumentation amplifier has an arrangement shown in FIG. 20.
The operation principle of this circuit will be described below. An input voltage .DELTA.VA (=VA1-VA2) is input across input terminals A1 and A2, and an output voltage .DELTA.VOUT (=(1+2R1/R2).DELTA.VA) proportional to the differential input voltage is generated across output terminals OUT1 and OUT2. In this case, R31=R32=R33=R34. In this circuit, however, when the input voltage range is narrow, and the resistance values of four resistors R31, R32, R33, and R34 vary, the center value of the output voltage varies in accordance with variations in center value of the input voltage.
The first problem with this circuit is that the variation range of the center value of the input voltage is limited. When R1/R2 is 100 or more, the voltage across nodes N3 and N4 becomes 200 times the input voltage or more. The nodes N3 and N4 vary in accordance with the variation in center value of the input voltage. Therefore, the voltage range of the nodes N3 and N4 is obtained by adding the variation range (.DELTA.VAcen) of the center value of the input voltage to a voltage obtained by multiplying a maximum value .DELTA.VA(max) of the input signal amplitude by an amplification factor.
More specifically, ##EQU1## Therefore, ##EQU2## The variation range of the center value of the input voltage is limited.
The second problem is that the output voltage .DELTA.VOUT varies in accordance with the center value of the input voltage. For a differential amplifier X3 to properly operate, R31=R33 and R32=F34 must hold. When the resistance values of R31, R32, R33, and R34 vary, the following equation (3) holds: EQU .DELTA.VOUT=K.DELTA.VIN+L(V4-VOUT2) (3)
for ##EQU3## As is apparent from equation (3), even when the input amplitude is zero, the output voltage amplitude varies in accordance with the input voltage.
To solve these problems, the output voltage must be divided and directly compared with the input voltage. To realize this arrangement, the DDA may be used as an instrumentation amplifier for an instrument. The DDA is represented by a symbol shown in FIG. 21, and its input/output relationship is given by: EQU (VOUT1-VOUT2)=A{(VA1-VA2)-(VB1=VB2)} (6)
where A is the amplification factor of the DDA.
FIG. 22 shows the arrangement of an instrumentation amplifier using a DDA.
In the example shown in FIG. 22, the input voltage .DELTA.VA is input across the terminals A1 and A2, and the terminal voltage at R2 is negatively fed back such that the terminal voltage equals .DELTA.VA. Therefore, a voltage -(2R1/R2+1).DELTA.VA is output across the terminals VOUT1 and VOUT2. Even when the center voltage of the input voltage varies, the output voltage does not change. For this reason, the input voltage range is wide. In addition, variations in resistors R1 and R2 only change the amplification factor. When the input voltage amplitude is zero, the output voltage amplitude is also zero.
The circuit arrangement of the DDA is described in detail in the following references [1] and [2]. However, both circuits have problems to be described below.
[1] Eduard Sackinger et al., "A Versatile Building Block--The CMOS Differential Difference Amplifier", IEEE, JSSC, Vol. SC-22, No. 2, April 1987, pp. 287-294.
[2] Shu-Chuang et al., "A Wide Range Differential Difference Amplifier. A Basic Block for Analog Signal Processing in MOS Technology", IEEE, Trans. on Circuits and Systems--II: Vol. 40, No. 5, May 1993, pp. 289-301.
FIG. 23 shows the circuit described in reference [1].
Two differential input voltages Al and A2 are received by two pairs of differential transistors M201 and M202, and M203 and M204, respectively, which have matched characteristics, and are converted into currents. The current difference is calculated by subtraction; the difference between the two differential input voltages is converted into a current difference.
A current difference .DELTA.I is given by: ##EQU4## for EQU .DELTA.VA=VA1-VA2 EQU .DELTA.VB=VB1-VB2 ##EQU5## where .mu. is the mobility of the MOS transistor, Cox is the capacitance per unit area of the gate oxide film, and W and L are the gate width and gate length of the transistor, respectively.
As is apparent from equation (7), the output current .DELTA.I is not proportional to the difference between the differential input voltages .DELTA.VA and .DELTA.VB. However, when the differential input voltages .DELTA.VA and .DELTA.VB have very small values, equation (8) below holds: ##EQU6##
When the amplitude of the differential input voltage is small, the circuit shown in FIG. 23 operates as a DDA. However, as the amplitude of the differential input voltage becomes larger, the linearity between the difference between the two differential input voltages and the output current is lost. This further decreases the gain, so the circuit ceases to function as a DDA.
FIG. 24 shows the circuit described in reference [2].
The output current (.DELTA.I=I1-I2) corresponding to the differential input voltage .DELTA.VA is given by: ##EQU7## where Kd is the current coefficient of M1 to M4, and Ku is the current coefficient of M5 to M8.
As can be seen from equation (9), even in the circuit shown in FIG. 24, the input voltage and the output voltage still have nonlinearity. This nonlinearity can be suppressed to some extent by minimizing Kd/Ku. However, the circuit shown in FIG. 24 has another problem that the circuit is influenced by the substrate bias effect. When the source of the transistor is connected to the substrate, the substrate bias effect can be suppressed although this poses a new problem of limitation on design and process.
As described above, in the conventional DDA, as the amplitude of the differential input voltage becomes large, linearity between the difference between two differential input voltages and the output current is lost, and the circuit is influenced by the substrate bias effect.